Unplasticized RTV-1 (room temperature vulcanizable, one part) compositions have unsatisfactory stress-strain behavior for use as permanently flexible sealing materials, since the stress values at, for example, 100% strain ("modulus at 100% strain") are too high. A high 100% modulus (e.g. 0.6 N/mm.sup.2) is associated with a marked increase in stress in the event of widening of the joint, caused for example by changes in temperature, which markedly increases the danger of adhesion failure, i.e. detachment of the flexible sealing material from the respective joint substrate. It has been found in practice, that a 100% modulus of from 0.2 to 0.4 N/mm.sup.2 is desirable for this type of joint.
The stress-strain behavior of a silicone rubber vulcanizate is a function of the type and amount of the filler and especially of the number of crosslinking nodes per unit of volume. The modulus of RTV-1 systems can be reduced by increasing the chain length of the hydroxyl-terminated diorganopolysiloxane component. However, the use of very long-chain polymers compromises the consistency of the RTV-1 system. Thus, if the consistency of the filled paste is to be soft and paste-like but also self-supporting, the viscosity of the organopolysiloxane cannot be too high, otherwise the result is a stiff, putty-like paste which is difficult to use.
For this reason, compositions are frequently made using blends of long-chain polymers and low-viscosity silicone plasticizers. This procedure is common practice in order to prepare a low-modulus product, and the use of such plasticizers for reducing viscosity is vital if the material is to be adequately processable. In addition, the use of a trimethylsilyl-terminated, short-chain organopolysiloxane as a plasticizer also reduces the number of crosslinking nodes per unit of volume, and therefore lowers the modulus.
The fundamental disadvantage of a plasticizer is that it is not chemically bound to the vulcanizate network. It can therefore migrate out onto adjacent surfaces and form deposit, either by being dissolved out from the vulcanizate by contact with a solvent, or by spontaneous migration. This phenomenon becomes particularly noticeable with joints in porous natural stone, such as marble, granite or altoquartzite, since the periphery of the joint becomes hydrophobicized and darkens therefore permanently, and this is of course undesirable. Products comprising plasticizers should therefore in principle not be used for filling joints in natural stone.
To avoid the addition of plasticizer in alkoxy RTV-1 compositions, EP-A-763 556 condenses a short-chained OH-terminated organopolysiloxane with catalysis by an acid phosphoric ester, e.g., di-2-ethylhexyl phosphate, forming a long-chained OH-terminated organopolysiloxane. The condensation is terminated by, for example, incorporating chalk after 14 minutes, which evidently brings about neutralization and thus deactivation. This is followed by the alkoxy end-capping of the long-chained OH-terminated organopolysilane with methyltrimethoxysilane, with catalysis by a titanium catalyst. Low-modulus vulcanizates can be prepared in this way. However, this process requires an excessive number of operations and gives unvulcanized compositions which are undesirably stiff.
U.S. Pat. No. 4,755,578 describes an alkoxy RTV-1 composition which comprises a blend of alkyldialkoxysilyl- and trialkoxysilyl-terminated polymers with dialkylmonoalkoxysilyl-terminated polymers. The dialkylmonoalkoxy end groups are not active in crosslinking. The preparation from hydroxyl-terminated diorganopolysiloxane and dialkoxysilanes, and also trialkoxysilanes, takes place over a relatively long period at relatively high temperatures, such as from 60 to 80.degree. C. The incorporation of other constituents into the RTV-1 composition has to take place separately afterward. The disadvantages of such preparation need not be stated.